1. Field of the Invention
The present invention relates to a cell search device and method for enabling a mobile station that uses a W-CDMA communication method to perform a base station search to identify spreading timings of a plurality of base stations located in a vicinity of the mobile station and spreading codes used by the base stations and differing for each base station.
2. Description of the Related Art
In recent years, spread-spectrum communication, which is relatively invulnerable to interference and disturbance, has received considerable attention as a communication method for use in a mobile communication system. This spread-spectrum communication is a communication system in which information signals to be transmitted are spread by means of a spreading code on the transmitting side and then transmitted. On the receiving side, de-spreading is carried out using the same spreading code as the spreading code to obtain the original information signals. The information signals to be transferred as information are referred to as symbols, and a unit of data that constitutes the spreading code for spreading these symbols is referred to as a chip.
In the spread spectrum communication system, a plurality of transmitters perform spreading by using different spreading codes having orthogonality, and a receiver selects the spreading code used when performing de-spreading, to thereby specify each communication. As a result, code division multiple access (CDMA) can be realized in which a plurality of communications use the same frequency band.
In the transmitting/receiving system of the spread spectrum communication method, however, de-spreading cannot be performed properly if the spread timing, which is the timing for performing spreading on the transmitting side, cannot be accurately obtained on the receiving side. For example, if even one chip of this spreading timing diverges, none of the signals that have been transmitted from the transmitting side can be received on the receiving side. As a result, a synchronization acquisition device is provided in a receiver in the CDMA communication method for obtaining the spreading timing of the transmitting side.
In addition, the use of the CDMA communication as a mobile communication system enables one mobile station to communicate with a plurality of base stations at the same time. When, for example, a mobile station is moving rapidly, base stations in the vicinity must be located in a short time in order to switch the base stations that complete a connection. A mobile station in a CDMA communication system must therefore not only search for one base station for current communication, but must also search for other base stations that are located in the vicinity. This process of searching for a plurality of base stations that are located in the vicinity is referred to as a cell search.
In the W-CDMA (Wide band-CDMA) communication method that is adopted in IMT-2000, which is the communication system of the next generation of portable telephones, an inter-base station asynchronous system is adopted in which signals are transmitted between each of the base stations without establishing synchronization. As a result, a process for searching for the spreading timing, which is a different timing for each base station, must be performed as the cell search on the receiving side in the W-CDMA communication method. As an example, a hierarchical cell search method in a spread-spectrum communication method is described in the Technical Reports of the Proceedings of the Institute of Electronics, Information, and Communication Engineers (IEICE), RCS (Radio Communications Systems) 99–154, 1999-11).
The downlink physical channel format that is handled in a cell search in a W-CDMA communication method is next described with reference to FIG. 1.
In the W-CDMA communication method, transmitted data are made up of units of 10-msec frames. These frames are in turn made up of 15 slots. The symbol rate per slot is 10 in the three types of physical channels that are used in a cell search: SCH (Synchronization Channel), CPICH (Common Pilot Channel), and P-CCPCH (Primary Common Control Physical Channel). All ten symbols are separately assigned for the CPICH. For SCH and P-PCCPCH, on the other hand, the space of the first symbol space of 10 symbols is assigned to SCH, and the remaining space of 9 symbols is assigned to P-CCPCH. Because this explanation is in regard to W-CDMA, explanation is given for a case in which one logic symbol bit is spread to 256 chips.
SCH is made up of a combination of p-SCH (primary SCH) and s-SCH (secondary SCH). Of these, p-SCH is stipulated in International 3GPP 25.213 “Spreading and modulation (FDD)” V3.3.0 to be spread by a Primary Synchronization Code (PSC), which is a spreading code used in common by all base stations and all slots. Channels other than p-SCH are not spread by PSC, and accordingly, performing de-spreading by PSC upon signals from a base station causes only p-SCH to appear as the original symbols. A cell search device takes advantage of this feature to detect the spreading timing. CPICH and P-CCPCH are spread by scrambling codes, which are spreading codes that differ for each base station, and therefore cannot be decoded without identifying the scrambling code of the base station that transmitted the received signals.
An example of the construction of a CDMA receiver that is provided with this type of cell search device is shown in FIG. 2. In an actual W-CDMA method, signals that are downlink from a base station undergo orthogonal modulation, but an explanation regarding the functions for orthogonal demodulation will be omitted in the interest of simplifying this explanation.
This CDMA receiver includes RF unit 1, A/D converter 2, correlator 3, and cell search device 94.
RF unit 1 demodulates signals that have been received from a transmitter and converts the signals to baseband signals. A/D converter 2 performs A/D conversion of the baseband signals that have been demodulated by RF unit 1 to convert the signals to digital data. Correlator 3 finds correlation values between the digital data that have been output from A/D converter 2 and predetermined spreading codes. As for the actual construction of correlator 3, in some cases a matched filter is used that holds spreading code length portions of digital data and simultaneously makes correlations with the spreading code, and in other cases a correlator bank is used that makes successive correlations between the spreading code and the digital data and accumulates these correlations.
As shown in FIG. 3, cell search device 94 is made up of averaging processor 41 and peak detector 43.
Averaging processor 41 performs an averaging process by averaging a correlation value profile from correlator 3 a fixed number of times for each timing. Peak detector 43 successively detects the peak values of the correlation value profile that has undergone the averaging process by averaging processor 41 and the peak timings at which the peak values have been obtained
FIG. 4 shows an example of correlation values for each timing obtained by correlator 3. In FIG. 4, actual data of the correlation values of the 10240 timings in the space of one slot are shown as an artificially created graph for the purpose of explanation.
In the W-CDMA method, quadruple oversampling is normally performed when calculating correlation values. In this case, the spreading length of PSC is 256 chips, the number of symbols per time slot is 10, and the number of timings, which is the amount of data of the space of one slot, is 4 (quadruple sampling)×256 (chips)×10 (symbols)=10240.
Cases in which correlation values indicate high values are those times when the timings of PSC and SCH coincide, and this graph includes a plurality of such timings. These cases indicate that the mobile station is receiving waves from a plurality of base stations.
This case shows correlation value data in the space of one slot, but in a mobile communication system that is made up of a mobile station and a plurality of base stations, correlation values will fluctuate widely due to the influence of such factors as fading because the mobile station communicates with base stations while moving. At times of a high degree of fading, the obtained correlation values will drop drastically, giving the impression that no waves are being received from the base station.
In order to overcome this type of problem, a slot averaging process is performed by averaging processor 41 in which the average of the correlation value data for the space of each slot is calculated instead of detecting the spreading timing based on the data of a single correlation value, thus preventing malfunctioning that is caused by loss of data due to fading.
Referring now to the flow chart of FIG. 5, a cell search process by cell search device 94 of the prior art is next explained.
First, in step 101, correlator 3 calculates a correlation value profile and an averaging process is carried out in averaging processor 41.
The peak value in the correlation value profile and the peak timing are next detected by peak detector 43 to detect the slot timing in step 102. Then, in step 103, this slot timing is used in the processes of detecting frame timing, identifying the code group, and identifying the scrambling code. Although the identification of scrambling code requires various processing steps, these processes will not be described here because they are not directly related to the present invention.
In step 104, a decoding process is carried out using the obtained scrambling code. In the decoding process, a CRC (Cyclic Redundancy Code) check is performed, and if an OK is obtained in this CRC check, the peak is confirmed to be a peak produced by a base station. If the CRC check results in a “NG,” on the other hand, the peak is judged to be the result of some type of noise and not a peak produced by a base station. Upon completion of the search for one base station, the processes of steps 102–104 are repeated to search for a second and third base station.
By carrying out a cell search by the above-described processes, a mobile station can learn the peak timings and scrambling codes of base stations that are located in the vicinity. In actual correlation value profiles that are obtained in the field, however, a variety of noise is generated in addition to the peaks of base stations, including peaks resulting from multipath reception that occur when waves from a base station are reflected from geographical features and buildings, cross-correlation peaks generated with other channels, and autocorrelation peaks generated with the same channel. In the correlation value profile shown in FIG. 4, for example, peak A is a peak caused by a first base station #1 and peak D is a peak caused by a second base station #2, but the other peaks are the result of these types of noise. If a cell search is performed using this correlation value profile, a judging process is carried out to determine whether or not each peak is a base station peak starting from the largest correlation value, with the result that a judging process is carried out to determine whether or not peaks A, B, C, and D are base stations in that order. However, since peaks B and C in FIG. 4 are actually the result of some type of noise and not peaks caused by base stations, the CRC check in the decoding process results in NG and these peaks are determined to be noise.
However, since these peaks cannot be determined to be noise until after the decoding process and CRC check, the judging process to determine whether or not a peak is produced by a base station must be performed for peaks that are caused by noise, and the time required for a cell search is consequently lengthened. Lengthening of the time required for a cell search leads to an increase in power consumption.
In the above-described cell search device of the prior art, the search for base stations is carried out based only on the size of peak values, and as a result, a judging process to determine whether or not a peak is caused by a base station must be performed even for noise such as multipath reception and autocorrelation peaks, and the prior art device therefore suffers from the problem that a cell search requires a long time to search for a fixed number of base stations.